1. Field of the Invention
This invention relates to a method and device for processing of granules of solidified salts.
2. Description of the Related Art
Particulate matter of solidified salts is widely used in many fields, i.e. as fertilizers in agriculture, additives in industrial process chemistry, de-icing aid of roads, etc. For many applications, consumers are in need of particulate matter with a specific particle size distribution and which is substantially free flowing after storing and transportation.
Fluidized bed spray granulation is a commonly used technique for producing homogeneous, evenly shaped granules originating from a liquid product or products, e.g. solutions, suspensions, slurries, melts and emulsions. However, fluidized bed spray granulation processes will usually yield granules out of the granulator with various sizes, where some of the granule sizes will be outside a desired size interval. The granules with acceptable sizes are often called on-size, i.e., corresponding to the size desired for the final product. The granules with sizes larger than desired are often called oversize, while the granules with sizes smaller than desired are often called undersize. On-size granules will be processed further, while undersize and oversize granules are typically recycled back to the granulator, eventually including handling and processing in the recycle loop.
Typically, the undersize granules might be directly fed back to the granulator (or after, e.g., cooling or drying if required by the process), in order to be further enlarged to reach the desired diameter range (on-size product); While the oversize granules will typically be crushed and then recycled to the granulator under solid form (possibly after some other intermediate process steps), or molten/dissolved to be recycled back to the granulator as liquid to be sprayed.
Product recycled back from the screens to the granulator under solid form will be hereinafter referred to as external recycling.
This recycling can sometimes be a necessity for process reasons, for example, to keep the system “in balance” (heat balance, water balance, etc). However, in most cases, this recycling is a result of non-ideal behavior of the granulation process and the fact that all particles do not have the desired size, and is therefore an unnecessary cost and a limitation of the process.
Generally speaking, recycling means higher investment costs and higher operating costs. Recycling requires processing, handling (conveyors, elevators, etc.), utilities consumption (electricity consumption because of, e.g., handling equipment; or heat consumption for, e.g., reheating the product to reach the desired temperature in granulator, etc.). Thus, the recycling should be minimized, which is one of the main advantages that this invention provides.
For example, in the case of granulation of urea for fertilizer purposes, customers typically want granules with diameters between 2 and 4 mm. Particles exiting the granulator with sizes below 2 mm will therefore be considered as undersize, while particles above 4 mm will be considered as oversize. Some oversize particles (lumps, for example) will be re-molten or dissolved, while most will be crushed to supply to the granulator necessary seeds for granulation. In such cases, external recycling is typically of 0.5:1 to 1:1, i.e., for 1 kg of on-size product exiting the granulation loop, between 0.5 to 1 kg of product (undersize and crushed oversize) are recycled back to the granulator.
Conventional continuous fluidized bed spray granulators will typically have a simplified process flow as indicated in FIG. 1 (auxiliary fluids, such as a fluidizing medium, utilities, etc., and their specific processing (e.g. heating, cooling, conditioning, cleaning, etc.) are not represented for simplification purposes). Reference numeral 1a represents a supply of small granules of the product or other seed particles (also called gem particles), if any, which are to be coated with the liquid product or products. 1b is the supply of the liquid product(s). Step 2 is a granulation zone where the seed particles are contacted with a spray of the liquid product and initially shaped and dried. Step 3 is further processing of the granules, which typically includes further drying and/or shaping and/or cooling of the granules. Step 2 and 3 constitute the granulation process and both may take place in a common fluidized bed. Step 4 is an optional process step prior to screening; for example, cooling, drying, etc. Step 5 is screening of the granules exiting the granulation process, 5a is the fraction of oversize granules, 5b is the fraction of undersize granules, and 5c is the fraction of on-size granules that are sent to step 6 for post-granulation processing to form the finished product 7.
Undersize granules 5b are sent from screening 5 to processing step 9 and recycled into injection zone 2. Steps 5/8/9 constitute the recycle loop. Oversize granules 5a are sent to processing step 8 to be recycled back to the injection zone. Step 8 is a treatment step of the oversize granules 5a. This treatment may be to make them smaller sized particles by crushing, to melt them to form liquid product to be sprayed into injection zone, or discharge them as large granules for external applications. Step 9 is an optional process step for processing the undersize granules 5b before re-entry into the seed stream.
As an example of a fluidized bed granulation process, please refer to a typical flow sheet of a urea granulation plant and urea granulator schematic representation in Fertilizer Manual, IFDC, Kluwer academic publishers, edition 1998, Ch 9, pp. 269 and 270, or in Nitrogen and Syngas, July-August 2006, p42.
Examples of fluidized bed spray granulation are given in, e.g., U.S. Pat. No. 5,213,820, and examples of fluidized spray granulation with a classifier are given in, e.g., WO 01/43861.
WO 01/43861 describes a fluidized bed spray granulator including a classifier for classifying the granules comprising a set of adjacent fluidized zones divided by inclined walls. The design of the granulation chamber is asymmetric, i.e., it is divided into several compartments by tilted baffle plates. The segregation is obtained by circular motion of the particles; the larger particles will tend to move towards the outlet while the lighter particles will tend to move towards the inlet of the bed, back to the growing zone where they can be enlarged. The document claims that all necessary recycling is made by the classifier, there is no external recycling.
WO 97/02887 discloses a fluidized bed classifier including a set of baffles in the bed to obtain vertical segregation when operating with gas velocities causing gas bubbles in the bed. The baffles have the effect of preventing particles from following the rising gas bubble up through the bed, and thus disturb the segregation effect.